1. Field of the Invention
The present invention is generally related to a small-cell mobile radio communication system such as a cellular mobile telephone system and particularly related to a method and apparatus for determining timing of a hand-off for permitting a mobile station to continue a telephone call while the mobile station is roaming across different small cells.
2. Description of the Related Art
Already-utilized prior art systems of determining timing of a hand-off are classified into the following two types:
(1) A first type of the prior art system is the system that the hand-off timing is determined on the basis of a ratio of a length of time in which a detected value of a signal-to-noise ratio (hereinunder abbreviated as an SNR) is below a predetermined threshold to the total length of time of detecting or measuring the SNR; and
(2) Another type of the prior art system is the system that the hand-off timing is determined on the basis of a ratio of a length of time in which a detected value of received signal strength input (hereinunder abbreviated as an RSSI) is below a predetermined threshold value to the total time of measuring the RSSI. A typical system of the first type (hereunder referred to simply as an SNR system) is the NORDIC mobile telephone system and further that of the second type (hereunder referred to simply as an RSSI system) is the small-cell mobile telephone system developed in the United States of America (hereunder referred to simply as the U.S. cellular system).
Turning now attention to FIG. 5, there is illustrated a block diagram of a conventional hand-off timing detecting system proposed by, for example, ASTRONET CORPORATION in the United States of America. In this figure, the reference numeral 1 designates a receiving antenna; 2 a high frequency amplifier; 3 a first mixer; 4 a first station-originating signal oscillator; 5 a first intermediate-frequency filter; 6 a second station-originating signal oscillator/second mixer; 7 a second intermediate-frequency filter; 8 a second intermediate-frequency amplifier/discriminator; 9 a noise detector; 10 an SAT signal detector; 11 a comparator; 12 a logical circuit/controller; and 13 an output terminal.
Next, an operation of this system will be explained hereinbelow. High-frequency signals received by the receiving antenna 1 are amplified by the high-frequency amplifier 2. Then, the amplified signals from the amplifier 2 are combined with signals from the oscillator 4 by the first mixer 3 to produce first intermediate-frequency signals. Desired waves are selected from the thus obtained first intermediate-frequency signals by use of the band filter 5 and are converted into second intermediate-frequency signals by the second station-originating signal oscillator/second mixer 6. The second intermediate-frequency signals undergo selecting process in the band filter 7. Further, the thus selected signals are converted into voice signals by the intermediate-frequency amplifier/discriminator 8 and then the voice signals are fed to an SNR detecting circuit which is composed of the noise detector 9, the SAT signal detector 10 and the comparator 11. The voice signals contain supervisory audio tone (hereunder abbreviated as SAT) signals in addition to ordinary signals representing voices. The SAT signals are used to monitor circuits, and three frequencies of waves (namely, waves of 5970 Hz, 6000 Hz and 6030 Hz) are used as the SAT signals in the U.S. cellular system. The SAT signal is detected by the signal detecting circuit 10. On the other hand, noises, of which frequencies are close to the frequency of the SAT signal, are detected by the noise detector 9. Thereafter, an SNR is evaluated by comparing a voltage level S of the detected signal to a voltage level N of the noise in the comparator 11. An output of the comparator 11 is at a level indicating "on" state thereof (hereunder referred to simply as "on" level" when the evaluated value of the SNR exceeds or equals a predetermined threshold (for instance, 20 dB), whereas the output thereof is at another level indicating "off" state thereof (hereunder referred to simply as "off" level) when being below the threshold. The logical circuit/controller 12 detects a ratio of a length of time, in which the output of the comparator 11 is at the "off" level, to the total time of the measurement of an SNR. If the detected rate exceeds the threshold, the logical circuit/controller 12 sends out a hand-off request by putting the output terminal 13 at a level indicating "ON" status of the SNR detecting circuit (hereunder referred to simply as "ON" status level).
On the other hand, the RSSI system employs an RSSI detecting circuit which substitutes for the above described SNR detecting circuit 9, 10 and 11. Further, in the RSSI detecting circuit by using the similar procedures as in the SNR detecting circuit.
As described above, each prior art system of detecting timing of a hand-off employs either the SNR system or the RSSI system. Comparison between these SNR and RSSI systems reveals the following fact. First, in case that adjacent two small-cells or zones are arranged to overlap with each other slightly or in part, the SNR system surpasses the RSSI system at performance of appropriately detecting timing of a hand-off because the SNR can detect disturbances caused by interference or the like. However, in case that the adjacent two zones overlap with each other in large part, the RSSI system surpasses the SNR system because the RSSI system can detect high RSSI. In spite of this fact there is provided no system which is capable of optimizing the timing of a hand-off in accordance with every type of zone arrangement. The present invention is made to obviate the above-described problems of the prior art.